Process for the preparation of 1,4,7,10-tetraazacyclododecane

ABSTRACT

1,4,7,10-Tetraazacyclodecane, a precursor for the synthesis of macrocylic chelating agents for metal ions such as gadolinium, is prepared efficiently as a highly pure product on an industrial scale.

The present invention relates to a novel process for the preparation of1,4,7,1-tetraazacyclododecane (I) comprising the steps represented inScheme 1.

More precisely, the present invention relates to a process for thepreparation of 1,4,7,10-tetraazacyclododecane (commnonly named Cyclen)alternative to the classical Richman-Atkins synthesis (see for exampleJ. Am. Chem, Soc., 96, 2268, 1974), at present industrially used for theproduction of compound (I) in the form of the sulfate salt.

1,4,7,10-Tetraazacyclododecane is the precursor for the synthesis ofmacrocyclic chelating agents for metal ions, as these chelating agentsform very stable complexes with such ions.

In particular, the complexes with the paramagnetic metal ions,specifically the gadolinium ion, of said chelaing agents can be used inthe medical diagnostic field through Nuclear Magnetic Resonancetechnique, otherwise troublesome due to the high toxicity of the freeion.

At present, two contrast media are commercially available, namelyDotarem^((R)) and Prohance^((R)), two gadolinium complexes the chemicalstructure of which is based on Cyclen, while others are still underinvestigation.

Therefore, it is important to work out a synthetic process for thepreparation of said “building block”, which is cost-saving andindustrially advantageous.

The process for the preparation of compound (I) should, therefore, beadvantageous both from an economical and environmental point of view,avoiding the preparation of amine tosyl derivatives, commonly used inthe conventional Richman-Atkins synthesis.

WO 97/49691 discloses the preparation of compound (I) by means of thesteps represented in Scheme 2, in which compound of formula (III),decahydro-2a,4a,6a,8a-tetraazacyclopent[fg]acenaphthylene is the keyintermediate for the formation of compound (I), and is obtainable bycyclization of the intermediate (IV),3H,6H-octahydro-2a,5,6,8a-tetraazacenaphthylene, in its turn preparedfrom triethylenetetramine and glyoxal;

In order to cleave the two carbon atom-bridge which characterizescompound (III), therefore to obtain (I), an oxidizing process has beendescribed which allows to transform (III) into oxidation products whichcan subsequently be hydrolysed and transformed into (I) by basichydrolysis.

Alternatively to the oxidative cleavage, WO96/28432 suggests the directhydrolysis of (III) with hydrobromic acid, or with hydroxylamine inethanol solution under reflux.

On the other hand, Italian patent application MI 97A 000982, in theApplicant's name, disclosed a convenient process for the preparation of(I) starting from (III), alternative to the above one, comprising ahydrolysis step in aqueous solution, at slightly acidic, neutral orslightly basic pH, with a primary diamine of formula (VI), representedin the following Scheme 3:

in which x ranges from 0 to 2 and Q is —CH₂CH(OH)CH₂—, —(CH₂)₂NH(CH₂)₂—or —[(CH₂)₂NH]₂(CH₂)₂ when x is 1 or Q is —CH₂— when x is 2.

Diethylenetriamine (DETA) is particularly preferred.

The reaction takes place in water, at a pH range from 5.5 to 9,preferably from 6 to 8, at temperatures from 60 to 100° C., in thepresence of 2-20 mol of diamine per mol of (III), under inert gasatmosphere or in the air, for 12-48 h.

After completion of the reaction, the solution is alkalinized with abase, such as sodium hydroxide, concentrated to small volume or to aresidue, and compound (I) is extracted with a suitable solvent, such astoluene, chloroform, butanol, amyl alcohol. The organic phase isconcentrated to a residue, to obtain the crude macrocycle (I), which isfinally recrystallized from toluene or ethyl acetate.

However, the advantages provided by the simple combination of the twoprocesses according to the following

to obtain a valuable synthetic route to compound (I) are unsatisfactory,on the contrary unexpected technical problems arise, thus making theirapplication on the factory scale difficult.

More particularly, isolation of compound (III) obtained by extractionwith hexane, as described in WO 97/49691, leads to a loss of productduring the concentration step of the reaction mixture, due partly totransport phenomena and partly to chemical degradation connected withthe presence of parasitic alkylating agents.

In fact, as the cyclization reaction is preferential but not selective,the reaction between compound (IV) and 1,2-dichloroethane also givesrise to parasitic alkylating agents, as a consequence of side reactionsof partial alkylation of said compound (IV), in amounts which cannot beignored when operating on a large scale. These products are likely toreact during the concentration step, thereby decreasing the yield incompound (III).

It has surprisingly been found that these problems can be overcome byisolating compound (III) in the form of a salt of a suitable inorganicacid.

Moreover, it has also been found that isolation of compound (I) from thereaction mixture in the form of the hydrochloride improves theindustrial applicability of the process without affecting the overallyield in compound (I), as the liberation of the salt of compound (I) isquantitative.

It is therefore the object of the present invention a novel process forthe preparation of compound (I) according to the following scheme 1:

which comprises the following steps:

a) condensation of triethylenetetramine (TETA) with glyoxal hydrate inwater or water-soluble solvents or mixtures thereof, at a temperatureranging from 0 to 5° C., in the presence of stoichiometric amounts or ofa slight excess of calcium hydroxide, to give the compound of formula(IV);

b) reaction of compound of formula (IV) with 1,2-dichloroethane, inamounts from 1 to 5 mol per mol of compound (IV), in dimethylacetamide(DMAC) and in the presence of Na₂CO₃, in amounts from 5 to 10 mol permol of compound (IV), adding NaBr in amounts from 0.1 to 2 mol per molof compound (IV) at a temperature from 25 to 150° C., to give thecompound of formula (III) which is isolated in the form of a salt of aninorganic acid selected from the group consisting of hydrochloric acidand phosphoric acid;

c) hydrolysis of compound (III) by reaction with diethylenetriamine inwater, at pH ranging from 5 to 9, at a temperature ranging from 90 to120° C., in the presence of 5-10 mols of diethylenetriamine per mol ofcompound (III), under inert gas atmosphere or in the air, for 12-48 h,recovering compound (II) as the tetrahydrochloride; and optionally

d) quantitative liberation of the base to give compound of formula (I).

Step a) is substantially effected as described in WO 97/49691.

Step b) is also carried out according to the method described in WO97/49691, but preferably following a modification as illustrated in thesubsequent Italian application MI 97 A000783.

In particular, in the process of the present invention, condensation ofcompound (III) is carried out with 3-5 mols of 1,2-dichloroethane permol of compound (III), in DMAC, in the presence of sodium carbonate, andwith the addition of NaBr as a catalyst in amounts from 0.1 to 2 mol permol of compound (III). The preferred conditions involve 3 mol of1,2-dichloroethane, 10 mol of sodium carbonate, and the addition of 0.5mol of NaBr.

It has unexpectedly been found, and it is object of the presentinvention, that after completion of the reaction and filtration of theinorganic salts, the above mentioned problems can be overcome byaddition of an acid which is both soluble in dimethylacetamide andyields a salt of compound (III) insoluble in said dipolar aproticsolvent.

Hydrochloric acid and phosphoric acid proved to be particularly suitablefor this purpose.

It has been found that, by using mixtures containing compound (III)suitably diluted with DMAC and adding an amount of 37% (w/w) HClequivalent to 2-4 mol/mol of compound (IV), preferably 2,4 mol/mol, aprecipitate forms containing about 95% of compound (III) present uponcompletion of the reaction.

A further improvement results by replacing 37% (w/w) HCl with 85% (w/w)H₃PO₄, which allows to reduce the solvent amount necessary to attain thealmost complete precipitation of compound (III) as a phosphate. Theresulting salt is a diphosphate.

In particular, precipitation tests carried out prove evidenced that aratio of 2 mol of H₃PO₄ per mol of starting compound (IV) is excellentfor precipitating compound (III).

The use of 85% (w/w) H₃PO₄ also involves the use of less H₂O comparedwith 37% (w/w) HCl (which has to be taken in consideration when DMAC isrecovered by fractional distillation).

In order to isolate compound (III) hydrochloride, it is preferable tooperate with a dilution of 6 L DMAC/mol of compound (IV), whereas in thecase of diphosphate it is possible to operate in a more concentratedsolution, i.e. 4.5 L DMAC/mol of compound (IV), thus decreasing thenecessary amount of solvent.

Step c) is the hydrolysis, or better the deprotection of compound (III),which is the glyoxal-protected form of compound (I), according to themethod described in the Italian patent application MI 97A 000982, withan amine capable of irreversibly displacing glyoxal. Diethylenetriamine(DETA) proved to be extremely productive for this purpose.

The presence of DETA, however, involves problems concerning the directisolation of compound (I) free base from the hydrolysis mixture, which,according to the teachings of said patent application, is carried out byaddition of a base until strongly alkaline pH, extraction with tolueneand crystallization in suitable temperature and concentrationconditions.

Following said procedure, as it will be illustrated in the Examples,when using pure compound (III), although the conversion (III)/(I) on thereaction crude is rather satisfactory, the yield in purified product isabout 70% due to DETA impurities which make a further crystallizationstep necessary.

The separation of DETA from the reaction mixture is therefore ofparamount importance for qualitative and quantitative purposes, whereasthe definition of a riproducible procedure in the conversion (III)/(I)should take into account that the starting material is a reaction crude.

It has surprisingly been found that the final isolation of compound (I)in the form of the tetrachloride allows to recover more than 95% of thecompound (I) resulting from the hydrolysis reaction, and it is extremelyselective towards compound (I) compared with DETA and reactionimpurities, yielding a highly pure product.

When necessary, the tetrachloride can be quantitatively converted intothe free base according to known methods, by reaction with aqueous NaOHfollowed by elimination of H₂O (for example by azeotropical distillationwith toluene), filtration of the salts and crystallization from toluene.

The residual compound (I) present in crystallization mother liquors canbe recovered as tetrachloride and recirculated without losses. Theconversion (I)*4HCl/(I) can therefore be carried out quantitatively.

The (III):DETA=1:5 molar ratio is the most productive in the conversion(III) (hydrochloride or phosphate)/(I). Moreover, the purification ofcompound (I) as tetrachloride is not affected by the DETA amount in thereaction.

This way, the problems observed with the cited Patent are avoided,resulting in a more suitable process for the industrial scale.

The following examples illustrate the best experimental conditions tocarry out the process of the invention.

EXPERIMENTAL SECTION

The following method was used for the gas-chromatographic analysis

Instrumentation Gas-chromatographic unit Hewlett- Packard series 5890 IIPlus equipped with autosampler series 7673 and unit HP-3365 ColumnHP-ULTRA 1, 25 m, int. diam. 0.32 mm, film 0.52 μm (cod. HP no. 19091A-112) Oven temp. program 1st isotherm at 150° C. for 0.5 min; ramp 10°C./min to 185° C.; 2nd isotherm at 185° C. for 0.01 min; ramp 20° C./minto 240° C.; 3rd isotherm at 240° C. for 2 min Injector Split (spiltratio 1:60) Splitting flow 72 mL/min Temperature 260° C. Split insert(HP art. 18740-80190) with glass wool (Chrompack art.8490) andstationary phase Chromosorb^((R)) W HP 80-100 mesh (Supelco art. 2-0153)Detection FID Temperature 290° C. Column flow 1.2 ml/min Transport gasHe₂ Injection 1 μl Sample concentration 10-20 mg/mL in H₂O

EXAMPLE 1 Preparation of Compound (I)

A) TETA Purification

A reactor fitted for the reaction is loaded, under nitrogen atmosphere,with 5 kg of crude TETA, then, keeping the system under stirring andunder nitrogen atmosphere, 800 g of deionized water are added in 8 minkeeping the system internal temperature below 45° C.

After the system has settled at 35° C., the reaction mass is added with1 g of pure straight TETA hydrate, keeping under stirring for 1 h, then10 L of toluene are added in 20 min. The reaction mixture is heated to40° C., then cooled to 25° C. in 30 minutes, keeping this temperaturefor 30 min. The precipitate is filtered through a septum, washed withtoluene and dried in a static dryer (30° C.) under vacuum (2 kPa) for 24h. 3.71 kg of the desired product are obtained.

Yield: 89% (on anhydrous) compared with the content in linear isomer inthe starting mixture.

GC assay: 98.22% (% Area)

H₂O (Karl Fischer): 20.75%

B) Preparation of Compound (IV)

A reactor fitted for the reaction is loaded, under nitrogen atmosphere,with 3.71 kg of straight TETA hydrate, 20 kg of H₂O and 2.9 kg ofcalcium hydroxide. The resulting suspension is stirred under nitrogenatmosphere and cooled to 0-5° C., then, while keeping the T of reactionat 0-5° C., is added with a 9% (w/w) glyoxal aqueous solution obtainedby mixing 2.9 kg of 40% solution with 10 kg of H₂O.

After completion of the addition, the mixture is kept at 5° C. for 1 h,added with 1 kg of celite previously washed with H₂O and left understirring for 15 min. Calcium hydroxide is filtered off. The filtrate isconcentrated with rotary evaporator under reduced pressure to a dryresidue.

The product is not subjected to purification and is used as it is forthe subsequent reaction.

Yield: 98.5% (on anhydrous)

GC assay: 95.5% (% Area)

H₂O (Karl Fischer): 0.24%

C) Preparation of the Compound (III) as Phosphate on Pilot Scale

The reactor fitted for the reaction, pre-heated at 40° C., is loaded,under nitrogen atmosphere, with a solution of 3.48 kg of compound (IV)(prepared as described in the previous step) in 80 L of DMAC, 11.6 kg ofNa₂CO₃:NaBr=10:1 (w/w) micronised mixture and 5.94 kg of1,2-dichloroethane. The resulting mixture is heated to 80° C. and keptat said T for 3 h, then cooled to 25° C. and filtered through a septum,washing the salts with 10 L of DMAC. The filtrate is loaded again in thereactor.

Keeping the internal T at 20° C. and under nitrogen atmosphere, 4.61 kgof 85% (w/w) H₃PO₄ are added drop by drop therein. The mixture isstirred under said conditions for 2 h, then left to stand overnight. Theprecipitate is filtered through a septum and washed with 10 L ofisopropanol. The product is then dried in a static dryer under vacuum toobtain 7 kg of crude compound (III) phosphate (content in (III)diphosphate: 65% w/w).

Yield: 58%

D) Preparation of Compound (I)

A reactor fitted for the reaction is loaded, under nitrogen atmosphere,with a solution of 7.0 kg of crude compound (III) phosphate in 14 kg ofH₂O, 5 kg of diethylenetriamine are quickly added thereto and theresulting mixture is adjusted to pH 7 by addition of 34% HCl. Theresulting mixture, stirred under nitrogen atmosphere, is refluxed andkept in said conditions for 24 h, then cooled to 25° C. and added with10 kg of 34% HCl. The resulting solution is concentrated under reducedpressure to a weight of 30 kg.

An equal weight of 34% HCl is added thereto, stirring for at least 2 hat 25° C., then the mixture is left to stand overnight. The precipitateis filtered and washed with 20% (w/w) HCl to obtain 4 kg of aprecipitate which is dissolved in 5 kg of H₂O at 60° C. Insolubles arefiltered off at said temperature, the solution is transferred into areactor pre-heated at 50° C., and 7.15 kg of 34% (w/w) HCl are added in1 h, keeping said T and stirring. The mixture is cooled to 20° C. andfiltered, washed with 20% (w/w) HCl and with absolute ethanol. Afterdrying in a static dryer under vacuum, 2.3 kg of crystalline compound(I) tetrachloride are obtained.

Yield: 36.1% (compared with compound (IV))

GC assay: 99.89% (% Area)

H₂O (K.F.): 0.18%

Acidic titer (0.1 N NaOH): 98.9%

Argentometric titer (0.1 N AGNO₃): 99.98%

Complexometric titer (0.1 N ZnSO₄) 98.6%

¹H-NMR, ¹³C-NMR, IR and MS spectra are consistent with the indicatedstructure.

EXAMPLE 2 Isolation of Compound (III) as Crude Hydrochloride

The preparation of compound (III) hydrochloride is carried outsubstantially as in step C) of Example 1, except that no final drying iscarried out. Concentrated hydrochloric acid is used instead ofphosphoric acid. At the end of the isolation, the humid product is notsubjected to drying but it is directly analyzed for the determination ofthe content in (III). The yields under various isolation conditions arereported in table 1.

TABLE I Isolation of compound (III) as crude hydrochloride. Compound(IV) Kg mol DMAC (L) mol HCl/(IV) % yield 0.64 3.9 23.4 2.41 46 2.5 1590 2.41 51 2.5 15 90 2.41 49

EXAMPLE 3 Isolation of Compound (III) as Crude Phosphate

TABLE II Compound (IV) Kg Mol DMAC (L) mol H₃PO₄/(IV) % yield 0.33 2 122.4 53 0.98 6 36 2.4 52 0.90 5.5 54 1 41 2.5 15 67.5 2 58 2.5 15 67.5 449

What is claimed is:
 1. A process for the preparation of 1,4,7,10-tetraazacyclododecane, said process comprising the steps of: a) condensing triethylenetetramine with glyoxal hydrate in water or water-soluble solvents or mixtures thereof, at a temperature ranging from 0 to 5° C., in the presence of stoichiometric amounts or of a slight excess of calcium hydroxide, to give the 3H,6H-octahydro-2a,5,6,8a-tetraazacenaphthylene; b) reacting 3H,6H-octahydro-2a,5,6,8a-tetraazacenaphthylene with 1,2-dichloroethane, in an amount from 1 to 5 mol per mol of 3H,6H-octahydro-2a,5,6,8a-tetraazacenaphthylene, in dimethylacetamide and in the presence of Na₂CO₃, in an amount from 5 to 10 mol per mol of 3H,6H-octahydro-2a,5,6,8a-tetraazacenaphthylene, adding NaBr in an amount from 0.1 to 2 mol per mol of 3H,6H-octahydro-2a,5,6,8a-tetraazacenaphthylene at a temperature from 25 to 150° C., to give decahydro-2a,4a,6a,8a-tetraazacyclopent[fg]acenaphthylene, adding an inorganic acid selected from the group consisting of hydrochloric acid and phosphoric acid, and isolating decahydro-2a,4a,6a,8a-tetraazacyclopent[fg]acenaphthylene in the form of a salt; c) hydrolyzing decahydro-2a,4a,6a,8a-tetraazacyclopent[fg]acenaphthylene by reaction with diethylenetriamine in water, at pH ranging from 5 to 9, at a temperature ranging from 90 to 120° C., in the presence of 5-10 mols of diethylenetriamine per mol of decahydro-2a,4a,6a,8a-tetraazacyclopent[fg]acenaphthylene, in an inert gas atmosphere or in air, for 12-48 hours, recovering the hydrolyzed product as the tetrahydrochloride by adding hydrochloric acid; and optionally d) quantitatively liberating the base to give 1,4,7,10-tetraazacyclododecane.
 2. The process as claimed in claim 1, in which condensation of decahydro-2a,4a,6a,8a-tetraazacyclopent[fg]acenaphthylene in step (b) is carried out with 3-5 mol of 1,2-dichloroethane per mol of decahydro-2a,4a,6a,8a-tetraazacyclopent[fg]acenaphthylene, in dimethylacetamide, in the presence of sodium carbonate, and with the addition of NaBr as a catalyst in an amount from 0.1 to 2 mol per mol of decahydro-2a,4a,6a,8a-tetaazacyclopent[fg]acenaphthylene.
 3. The process as claimed in claim 2, in which 3 mol of 1,2-dichloroethane, 10 mol of sodium carbonate and 0.5 mol of NaBr are used.
 4. The process according to claim 1, 2 or 3, in which the final reaction mixture from step (b) is added with an amount of concentrated HCl equivalent to 2-4 mol/mol of 3H,6H-octahydro-2a,5,6,8a-tetraazacenaphthylene.
 5. The process as claimed in claim 4, wherein the concentration of the solution is 6 L of dimethylacetamide/mol of 3H,6H-octahydro-2a,5,6,8a-tetraazacenaphthlene.
 6. The process according to claims 1, 2 or 3, in which the final reaction mixture from step (b) is added with an amount of 85% H₃PO₄ equivalent to at least 2 mol/mol of 3H,6H-octahydro-2a,5,6,8a-tetraazacenaphthylene.
 7. The process as claimed in claim 6, wherein the concentration of the solution is equivalent to 4.5 L of dimethylacetamide/mol of 3H,6H-octahydro-2a,5,6,8a-tetraazacenaphthylene.
 8. The process according to claim 1, in which in step (c) the molar ratio of salt of decahydro-2a,4a,6a,8a-tetraazacyclopent[fg]acenaphthylene to diethylenetriamine is 1:5. 